A logo was a symbol or emblem used to promote a brand or organisation.
PhiCorp used the letter Phi as its logo. (TV: The Gathering)
The Fifth Doctor recalled that his book series was to be named with a comma, The Doctor, Who Discovers but became Doctor Who Discovers after "some idiot" in the publisher's design department missed the "The", creating a logo that said Doctor Who. (AUDIO: The Kingmaker)
The Torchwood Institute used the letter T as its logo. (TV: Army of Ghosts [+]Russell T Davies, Doctor Who series 2 (BBC One, 2006). etc.)
The Unified Intelligence Taskforce used multiple logos throughout its history. Most of them resembled the planet Earth with a pair of wings. (TV: The Sontaran Stratagem [+]Helen Raynor, Doctor Who series 4 (BBC One, 2008)., The Day of the Doctor [+]Steven Moffat, 50th Anniversary Specials (BBC One, 2013)., The Star Beast [+]Russell T Davies, adapted from Doctor Who and the Star Beast (Pat Mills and John Wagner), Doctor Who 2023 specials (BBC One and Disney+, 2023). etc.)
Other realities
Cybus Industries used the letter C as its logo. This logo appeared on most of its products, such as the Cybus EarPods. The Cybermen created by Cybus Industries also had the logo prominently displayed on their chests, which prompted the Tenth Doctor to remark that John Lumic was turning them "into a brand". (TV: The Age of Steel [+]Tom MacRae, adapted from Spare Parts (Marc Platt), Doctor Who series 2 (BBC One, 2006). etc.)
Behind the scenes
Information from invalid sources
The TARDIS once flew through the Doctor Who logo, leaving a line behind it, striking through the logo. (NOTVALID: The NEW Doctor Who Logo)